A conventional image sensor array is typically formed from a plurality of photosensitive elements or pixels arranged in rows or columns. FIG. 8 illustrates an example of a typical PIN (P+/Intrinsic/N+ layered) photosensitive element 10 used in the image sensor array.
Each photosensitive element 10 includes a contact pad 14 positioned over a substrate 11. A photosensor island 12 of doped amorphous silicon (a-Si), includes a P+ doped region 24 and an N+ doped region 22 that covers contact pad 14. Photons that enter photosensor island 12 generate electrons in the a-Si. An applied voltage generates an electric field between transparent upper conductive layer 16 of indium tin oxide (ITO) and contact pad 14. The electric field moves the generated electrons to contact pad 14. Passivation layer 18 separates transparent upper conductive layer 16 from substrate 11 except where the ITO contacts an upper surface of phosphor island 12. Passivation layer 18 typically includes an oxynitride layer 26 and a polyamide layer 28.
Conventional image arrays that use the photosensor element illustrated in FIG. 8 include spaces between adjacent elements. These spaces do not detect light. A ratio of element areas that detect light to space areas occupied by the pixel is defined as a pixel fill factor. Method's of defining fill factor are described in an article entitled "High Efficiency X-Ray Imaging Using Amorphous Silicon Flat-Panel Arrays" by J. Rahn, F. Lemmi, J. P. Lu, P. Mei, R. B. Apte, and R. A. Street published in IEEE Trans. Nucl. Sci. (USA), IEEE Transactions on Nuclear Science (June 1999) vol.46, no.3, pt.2 p. 457-61 and hereby incorporated by reference.
High fill factor image arrays greatly improve the pixel fill factor such that an increased area of the sensor array detects light. FIG. 9 illustrates a high fill factor image array 40 that uses a continuous P+ doped amorphous silicon layer 52 deposited over a continuous intrinsic amorphous silicon layer 50. The continuous layers allow light detection across the entire sensor surface.
A voltage difference between upper electrode 54 and a plurality of source-drain metal contacts 44 on substrate 42 creates an electric field through amorphous silicon layers 50. Upper electrode 54 is typically made of a transparent ITO while source-metal drain contacts 44 are made of an electrically conductive material such as a tri-layer TiW/Al/Cr. The electric field moves the generated electrons to contacts 44. Each contact communicates with switching and processing circuits (not shown) that generate an image based on the charge on each contact.
Patterned back contact collection electrodes 46 coupled to each source-drain contact 44 increases the area of electron collection. An N+ doped amorphous silicon layer 48 is deposited over each source-drain contact 44 to form a PIN structure with continuous layers 50, 52. A conventional passivation layer, typically an approximately one micron thick oxynitride layer, serves as an insulator between adjacent mushroom electrodes.
One problem with conventional image arrays are leakage currents that arise due to material defects. Leakage currents include lateral leakage current between adjacent mushroom contacts and vertical or intrinsic leakage currents that occur along the direction of arrow 58. Lateral leakage currents reduce image resolution. A typical 60.times.60 square micrometer of PIN sensor may include up to 0.3 pico-amps (pA) of lateral leakage current. A system for minimizing lateral leakage current is described in a patent application entitled Dual Dielectric Structure for Suppressing Lateral Leakage Current in High Fill Factor Arrays by Jeng Ping Lu, Ping Mei, Francesco Lemmi, Robert Street and James Boyce, Ser. No. (D/99215) 09/419293, hereby incorporated by reference.
Vertical leakage current also degrades image quality by introducing noise. The introduced noise reduces image contrast and/or gray scale. A typical 60.times.60 square micrometer of a PIN sensor may include about 20 femto-amps (fA) of intrinsic leakage current at five volt contact voltages.
Thus a method and apparatus for reducing vertical leakage current is needed.